RAILWAY CONSTRUCTION. 169 



down, and when the work is completed they are filled in again 

 and closed. These service shafts are generally made square in 

 section, and are merely lined with wood. Strong vertical 

 timbers are placed at the four corners, to which horizontal double 

 cross-pieces are bolted, thick planking being placed vertically at 

 the back of these cross-pieces to support the sides of the 

 excavation. 



The heading of a tunnel is a narrow passage or gallery cut 

 through from end to end of the works in the direction of the 

 centre line. Where there are shafts, the cutting of the heading 

 can be pushed on from several points, and be completed much 

 more rapidly than when the working is restricted to the two 

 ends. Headings are usually made just sufficiently large for the 

 miners to work, say about 5 feet 6 inches high by about 3 

 feet wide, the object being rather to expedite the driving of the 

 driftway than to remove large masses of material. They must 

 be set out with great accuracy, and be constantly checked as the 

 driving is in progress. When completed from end to end, the 

 centre line can be checked throughout, and the course actually 

 taken compared with the course intended. If there has been 

 much variation in the narrow pioneer pathway, either in line or 

 level, the amount of the divergence must be rectified when 

 ranging the final centre line for the full-size excavation. 



Tunnels cannot always be delayed until the heading is cut 

 through for the entire length. In many cases the heading, the 

 full-size excavation, and the permanent lining have all to be 

 carried on at the same time, but as the work of the heading is 

 smaller in extent, that portion of the operations can usually be 

 kept well in advance of the others. The critical moment arrives 

 when the headings from opposite directions meet, as any 

 deviation or want of coincidence must be adjusted in the portion 

 of the tunnel still remaining to be opened out to full size. Some 

 tunnels of moderate length have been constructed without any 

 heading at all, the excavation being taken out to the full 

 dimensions from the commencement. 



The heading of a tunnel assists not only in the correct 

 alignment of the work, but furnishes at the same time an 

 accurate knowledge of the strata passed through. It is also of 

 service for ventilation, communication, and drainage. 



In some cases the heading is driven at the bottom of the 

 tunnel section, as in Fig. 211, and in others at the top, as in 



